When I suggested, following Louise Mensch's resignation, that Labour would walk to victory in the Corby by-election, the party's deputy chair Tom Watson cast doubt on my prediction, insisting that "Corby will be a very tough fight".

But Watson needn't have managed expectations. With just over three weeks to go until polling day on 15 November, Lord Ashcroft's second poll of voters in the constituency, conducted for ConservativeHome, suggests that Labour is on course for a landslide victory. Since Ashcroft's last survey, the party's lead over the Tories has risen by seven to 22 points, with Labour on 54 per cent (up from 39 per cent at the general election) and the Conservatives on 32 per cent (down from 42 per cent).

As Tim Montgomerie suggests, the poll is notable for demonstrating how a collapse in the Lib Dem vote at the next election could hurt the Tories the most. Support for the party in Corby has plummeted from 15 per cent in 2010 to just five per cent now, with the bulk of Lib Dem supporters defecting to Labour. If this trend is replicated in other seats, Ed Miliband's party can expect to pick up dozens of Con-Lab marginals. While the Tories are in second place in most Lib Dem seats (38 compared to 17 for Labour), they will struggle to make gains if, as expected, the Lib Dems benefit from an incumbency effect (the party's MPs are famed for their constituency work).

But for Labour, the omens are more encouraging. In seats where it is within touching distance of the Tories (and even some where it is not), a collapse in support for the third-placed Lib Dems will likely propel it into first place.